In general, an alloy of the dispersion type such as a cermet in which the matrix of a metal contains a powder of a nonmetal or a powder of a different metal in uniform dispersion, is superior in the strength and the electro-chemical properties to an alloy of the melting type (i.e., a cast alloy) manufactured by the conventional melting process, and is therefore actively utilized in such special fields as a nuclear reactor fuel.
In the manufacture of an alloy of the melting type by the conventional melting process, however, it is particularly difficult to achieve uniform dispersion, in the matrix of a metal, of a powder of a nonmetal or a powder of a different metal having a largely different specific gravity from that of the metal forming said matrix. It is therefore usual practice in most cases to manufacture an alloy of the dispersion type mentioned above by the powder metallurgy process as a sintered alloy.
As is well known, the manufacture of a sintered alloy by the powder metallurgy process comprises sufficiently pulverizing and mixing raw material powders of elements constituting an alloy to be manufactured; pressing said raw material powders thus mixed to form a green compact of desired size and shape; and sintering said green compact. In an attempt to improve the sintering property and achieve a uniform chemical composition, a composite powder formed by covering the surface of each particle of a metallic powder or a nonmetallic powder with a different metal or a different nonmetal by a chemical, electro-chemical or mechanical method has become applied popularly, as a raw material powder for a sintered alloy, and the demand for the composite powder is only increasing.
There are conventionally known the following processes for manufacturing the aforementioned composite powder.
(a) A process comprising covering the surface of each particle of a metallic powder with a different metal by causing chemical precipitation of said different metal onto said surface;
(b) A process comprising covering the surface of each particle of a metallic powder or a nonmetallic powder with a different metal by electroplating said surface with said different metal;
(c) A process comprising covering the surface of each particle of a metallic powder with a different metallic powder or with a nonmetallic powder by grinding and mixing two different metallic powders in a ball mill or by grinding different metallic and mixing the metallic powder and the nonmetallic powder in a ball mill; and
(d) A process, as an application of the hydrogen pressure reduction process, comprising covering the surface of each particle of a metallic powder with a different metal after activating said surface with the use of such a surfactant as anthraquinone.
However, the processes (a) and (b) mentioned above pose environmental control problems in the disposal of waste liquid, and the process (c) mentioned above usually requires a long time for manufacturing a composite powder. Also, the process (d) mentioned above, which is being adopted as a process for economically manufacturing a composite powder for certain alloys, causes pollution in the disposal of waste liquid. Furthermore, all the processes (a) to (d) mentioned above are identical in the basic step of using a metallic powder or a nonmetallic powder prepared in advance by a known method as the starting material powder and covering the surface of each particle of said powder with a different metal or with a different nonmetal, and hence cannot be a simple process for manufacturing a composite powder.